As MOS integrated circuits have become more complex, circuit performance advances that normally result from the decrease in individual circuit component size with scaling has in recent years not been realized to its fullest extent. These advances have been limited by the RC time constant characteristics of the long interconnects required to interconnect the increased number of circuit components. To reduce the resistance component of these long interconnects, integrated circuit manufacturers have been turning to refractory metals, refractory metal silicides, and polysilicon/ refractory metal silicide composite films (referred to as polycides) to replace the polysilicon conventionally used for gate-level interconnects. Such materials offer interconnect sheet resistances of 1-3 Ohms per square, compared to 10 Ohms per square or more for polysilicon alone.
In addition to lower resistance, one advantage provided by polycides is the enabling of higher temperature processing subsequent to their deposition. This is necessary when attempting to form a high quality interlevel oxide to serve as an insulating layer between the various metal layers which, in present technology, can exceed three to four layers of interconnects. Typically, such oxide layers require the use of a deposited glass that is reflowed at temperatures of between 800.degree.-900.degree. C. Conventional metals such as aluminum, etc., will melt at this temperature. The use of a polycide provides a relatively stable material with a high melting point that will not go through phase changes during formation of the interlevel oxide.
During the design phase of a given process, there are a number of polycide films from which to select. Refractory metals such as titanium, tungsten, molybdenum, tantalum, etc. have been utilized to form the refractory metal silicide. Although all of these metals provide good thermal and chemical stability, they vary somewhat in their resistive properties. The most desirable refractory metal silicide from the standpoint of circuit operation is titanium silicide, which silicide exhibits the lowest electrical resistivity. In fact, titanium silicide is one of the few refractory metals that exhibits a lower electrical resistivity than the elemental metal from which the silicide is formed.
One disadvantage of titanium silicide is that it is not as resistant to oxidation as the other silicides. Therefore, any subsequent oxidation on the surface of the titanium silicide after formation thereof can result in a higher contact resistance for interconnections made from different levels to the silicide, which is a common problem that plagues most processing systems. Of course, use of silicides with much higher oxidation resistance and better chemical stability will solve this problem but these silicides do not exhibit the same resistance characteristics as titanium silicide. Therefore, there exists a need for improving the oxidation resistance of the silicide layer over that exhibited by titanium silicide without suffering any loss of conductive properties and chemical stability of the surface of the polycide.